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List SMT xx
On power 8 with AIX 7.1 you can have SMT8, on power 7 or AIX 6.1 only SMT4.

# lsattr -El proc0
frequency     3525000000
smt_enabled   true
smt_threads   8
state         enable
type          PowerPC POWER8

In order to change SMT, verify in the partition profile on HMC in processor section the compatibility mode. If power7 then you can have a maximum of SMT 4

# smtctl
This system is SMT capable.
This system supports up to 8 SMT threads per processor.
SMT is currently enabled.
SMT boot mode is set to enabled.
SMT threads are bound to the same virtual processor.

proc0 has 4 SMT threads.

Change SMT:

# smtctl -m on
# smtctl -t 8 -w boot
# bosboot -ad hdisk0